An arrangement of this kind is disclosed in the French Pat. No. 1,372,018. In this arrangement, the reserve fluid container is permanently secured in a hole in a fluid tank. A cap is intended to be put onto the upper part of the container to close the tank. A valve is provided to open and close respectively an opening in the lower part of the container. In the mantle of the container, there are provided openings continuously open into the tank. When fluid is filled into the hole in the tank, the fluid will flow down into the container and when the container is filled the fluid will flow out through the openings in the mantle and into the tank. This arrangement is unsuitable for certain applications, e.g. for portable power saws used in the forest industry. Manufacturers of power saws constantly aim at keeping the weight of the power saws as low as possible and a.o. the saws are provided with comparatively small fuel tanks, which have to be refilled frequently. A forest worker is often occupied with the felling of a tree when the fuel in the power saw is consumed and the same stops. At that time, the tree may be sawn off half ways or more. The forest worker must now refill the fuel tank and to do this he must move with the saw to a fuel supply, which may be located at a considerable distance from said tree. When the forest worker is moving to and away from the fuel supply and refilling the saw, the tree is left partially sawn off without observation and a puff of wind may be sufficient to get the tree to fall. Other workers near the tree or the first mentioned forest worker himself may accordingly be the subject of an obvious risk of getting hurt or killed. The arrangement according to said French patent specification is unsuitable to be used on a portable power saw since it is not possible for the worker to conveniently establish whether the container is capable of retaining the reserve fuel. Even if the valve is only slightly untight, a leakage will occur and it may happen that no reserve fuel is at hand in a critical situation. Furthermore, the reserve fuel in the container will simply flow out of the container through the openings in the mantle if the tank would be inclined or inverted.